CONCLUSION TO THE SPECIAL ISSUE ON THE NATIONAL SOCIAL COST OF CARBON

Abstract

The results of five Integrated Assessment Models are discussed in this concluding paper. All the scenarios measure the National Social Cost of Carbon (NSCC) across approximately 200 countries in a scenario with no mitigation. Despite assuming similar population and economic growth rates (RFF-SPs), the five models imply a wide range of forecasted climate impacts. Summing the mean NSCC estimates of all countries leads to a mean global Social Cost of Carbon (SCC) that varies from $33/tCO 2 to $1373/tCO 2 across the models. Most of the literature on climate IAMs has focused on these SCC values. This special issue, in contrast, is focused on the NSCC. However, the assumptions in each IAM model that led to different SCC values also determine the cumulative magnitude of the NSCCs from each model. The discount rate, the climate damage function, and the modeling of uncertainty strongly influence the magnitude of the NSCCs across models. What is unique to the analysis of the NSCCs is how they are distributed across countries. The NSCC values explain how climate damages and, therefore, the benefits of mitigation are distributed across nations. The IAMs predict that China and especially the United States are particularly exposed to climate change due to their large asset bases. About half of the global damage is assumed to happen in the 10 largest countries in the world, measured using GDP PPP. Most of the models predict every country has some damage, but one model predicts 29 countries — mostly in high latitudes — will benefit from near-term climate change. Regression analysis confirms that the distribution of climate damages across countries is primarily determined by the total size of the national economy. Because IAMs scale physical risks by economic exposure, they predict that two-thirds of the global climate burden is concentrated in the mid-to-high latitudes, where the majority of global economic activity is located. Finally, the IAMs predict a very wide range of NSCC values in 2100, ranging from a 40% increase to an eight-fold increase over current NSCC values, depending on structural assumptions regarding adaptation and catastrophic risk.